To be sure, Delbanco leaves no doubt about what the abolitionists achieved. "The contribution of the abolitionists was to make thinkable what had once been unthinkable, namely, black freedom," Delbanco writes, pointing to historian Eric Foner's assessment in his acclaimed 2010 book The Fiery Trial: Abraham Lincoln and American Slavery. "By pushing beyond conventional ideas about race and slavery, they changed both Lincoln's private judgment and public opinion, thereby vastly enlarging what was politically possible in nineteenth-century America." You won't find a better description of what the climate movement might hope to achieve — if in place of "race and slavery" you substitute fossil fuels and climate.

But Delbanco's major point, what his critics seem to find most provocative, is that it's entirely possible to give the abolitionists their full due, yet still sympathize with the "intellectual and political leaders who, although disgusted by slavery, nevertheless tried to forestall the catastrophic war they feared was coming." Indeed, as historian David Brion Davis points out in The New York Review of Books, the immense carnage of the Civil War, which scholars now believe caused on the order of 750,000 military deaths, "has long cast a backward shadow on the American abolitionists." (Davis adds: "Of course it should be stressed that without the abolitionist movement, there would have been no possibility of slave emancipation in the nineteenth century, and that it was Southern proslavery expansionism that brought on the war.")

It's this "shadow" that Delbanco, as a literary scholar, is so interested in probing. Nathaniel Hawthorne and his friend Herman Melville — who described slavery as "a sin . . . no less— a blot, foul as the crater-pool of hell," but despaired that "Not one man . . . knows a prudent remedy" — were both repelled by the abolitionists' extremism because, it seems, they didn't want the blood of a cataclysmic war on their hands. "They sensed," Delbanco tells us, "that Armageddon was coming — and that, if abolitionists and fire-eating slaveholders had their way, it would come soon." Both writers were "sensitive to the crime of slavery but squeamish about the abolitionist response." Most strikingly, Melville's monomaniacal Ahab in Moby-Dick was seen as "a timely personification of the zealotry that was rising, in 1850-51, on both sides of the slavery divide." Delbanco notes that contemporary readers saw in Ahab both William Lloyd Garrison and Southern senator John C. Calhoun.

Delbanco wants us to be alert and sensitive to this kind of moral complexity, and empathetic toward those who were sincerely conflicted about pushing too hard, too fast. The "sacred rage of abolitionism," he writes, "has been at work in many holy wars since the war against slavery." And so Delbanco would hold us back "from passing easy judgment on those who withheld themselves from the crusade, not out of indifference, but because of conscientious doubt."

I'd like to think that if I'd been a contemporary of Hawthorne and Melville — not to mention Douglass, Garrison, and Thoreau — that I would have had the courage and moral clarity to be among the abolitionists. But the truth is, no matter how virtuous I want to believe myself, I simply don't know. Nor, if we're honest, do any of us.